The high affinity receptor for IgE ("E-receptor") on mast cells plays a central role in immediate hypersensitivity reactions. Reaction of receptor-bound IgE with polyvalent antigen clusters the receptors and this stimulates a variety of cellular responses characteristic of allergic and inflammatory reactions. We study the molecular mechanisms by which E-receptors generate such responses. During the past year, our principal studies have been along the following lines: 1) We completed studies that analyzed the interaction of Lyn kinase with E-receptors permanently transfected into Chinese Hamster Ovary cells. These studies employed chimeric proteins consisting of the ecto- and transmembrane domains of an irrelevant receptor chain (alpha chain of IL-2 receptor) fused to portions of the E-receptor or of Lyn kinase. Our results showed that Lyn interacts with both the unphosphorylated beta and gamma chains of the receptor but preferentially with the former. This interaction occurred outside the so-called detergent-resistant "microdomains". Neither the myristoylation nor palmitoylation of Lyn was required to initiate the phosphorylation of the receptor. This will simplify identifying the critical amino acid residues engaged in this interaction. 2) We had previously examined the role of phosphatases in regulating the concentration of phosphorylated receptors with and without stimulation and had observed no change in phosphatase activity with stimulation. In recent studies we explored whether aggregation of the receptors, which induces their shift to the microdomains, thereby creates a protected environment for the phosphorylated receptors. Our results showed no evidence for such a phenomenon. 3) The spontaneous phosphorylation of E-receptors that is characteristically seen in the transfected cells is of interest because it suggests there may be factors regulating this crucial first step of the response that remain undiscovered. We substantially completed studies on this phenomenon. A large number of potential explanations have been eliminated and so far the only abnormal finding is that the alpha subunit of the transfected receptors is hyperglycosylated. Work from other laboratories suggests that glycosylation can alter the degree of spontaneous association of membrane proteins but so far we have been unable to observe evidence for such spontaneous aggregation using a variety of methods. 4) Our earlier studies showed that signaling by the receptor was subject to "kinetic proofreading". In such a regimen the signaling is controlled by the lifetime of the initial ligand: receptor interaction. We have now found that under conditions favoring the persistence of signaling (see (5)), even ligands that interact relatively weakly can stimulate late events such as degranulation and synthesis of mRNA coding for various mediators. We are exploring the basis for this apparent paradox. In a second study under this topic, we are examining whether the sequential phosphorylation of tyrosines on a critical early component differs for the high affinity and low affinity ligands. 5) Stimuli generating large-scale clustering of the receptors are short-lived. Our new studies suggest that entanglement of the aggregated receptor with cytoskeletal structures leads to such rapid "down-regulation" of activated receptors whereas a variety of procedures that limit such entanglement can prolong the signaling. The similarity of E-receptors to other receptors of the immune system (e.g. the clonotypic receptors on T and B lymphocytes), make it likely that the significance of our studies extends beyond the IgE/mast cell system.